Slice multiplexing method and apparatus for magnetic resonance imaging

Abstract

In a magnetic resonance slice multiplexing method and apparatus, measurements are performed repeatedly subject to the assignment of additional phases to the respective slices, the additionally assigned phases being changed with reach repetition such that at least one central k-space region is sampled completely in each of the repeated acquisitions. A calibration dataset is determined from the measurement data acquired completely in the central k-space region. The calibration dataset is used when reconstructing image data for the simultaneously excited slices from the acquired measurement data.

The invention claimed is: 
     
       1. A method for magnetic resonance (MR) imaging of at least two non-overlapping slices of an examination object, said method comprising:
 from a computer, emitting control signals to an MR data acquisition scanner in order to cause the MR data acquisition scanner to execute an MR data acquisition sequence in which acquired MR data are entered into a memory organized as k-space along a k-space trajectory, and in order to radiate at least one multiband radio-frequency (RF) excitation pulse that selectively and simultaneously excites nuclear spins in at least two non-overlapping slices of an examination subject situated in the MR data acquisition scanner; 
 in said computer, determining phases that are to be additionally assigned to each of the simultaneously excited slices during execution of said MR data acquisition sequence, so as to displace at least a first of the measurement points for a respective slice to be displaced away from said k-space trajectory of the MR data acquisition sequence, in a direction in k-space corresponding to the additionally assigned phase for that respective slice; 
 using said computer to operate said MR data acquisition scanner to execute said MR data acquisition sequence with the assignment of the determined additional phases in order to acquire MR data from the respective slices with at least two RF reception coils; 
 using said computer to emit control signals to operate said MR data acquisition scanner in order to repeat acquisition of said MR data with said at least two RF reception coils in said MR data acquisition sequence with the phases that are additionally assigned being changed from repetition-to-repetition so that, compared with a previous acquisition of said MR data, at least a second of the measurement points in a respective repetition of the MR data acquisition sequence, that have not yet been displaced away from said k-space trajectory in previous repetitions, are displaced away from k-space trajectory in said direction in k-space, with said repetitions of said MR data acquisition sequence being repeated with a number of repetitions that causes at least one central region of k-space to be sampled completely by the repeated MR data acquisitions; 
 in said computer, determining a calibration dataset from the measurement data acquired in said central region of k-space; and 
 in said computer, reconstructing image data for the simultaneously excited slices from the acquired measurement data using said calibration dataset, and making the image data available from the computer in electronic form, as a data file. 
 
     
     
       2. A method as claimed in  claim 1  comprising, in said computer, determining a different phase that is to be additionally assigned for each of the simultaneously excited slices. 
     
     
       3. A method as claimed in  claim 1  comprising assigning the additional phases to the respective slices by emitting control signals from said computer to said data acquisition scanner that cause at least one of activation of additional gradients, and manipulation of phases of individual RF pulses of which said multiband RF excitation pulse is comprised. 
     
     
       4. A method as claimed in  claim 1  comprising assigning said additional phases according to a CAIPIRINHA method. 
     
     
       5. A method as claimed in  claim 1  comprising determining said k-space trajectory so as to be in a plane in k-space, and determining said direction in k-space, in which said measurement points are displaced by the assigned additional phase, so as to be perpendicular to the plane of said k-space trajectory. 
     
     
       6. A method as claimed in  claim 1  comprising, in said computer, determining a predetermined shift in the image domain, in which said image data exist, and determining said phases to be additionally assigned so as to produce said predetermined shift in the image domain by the acquired measurement data. 
     
     
       7. A method as claimed in  claim 6  comprising determining said predetermined shift in the image domain based on a number of said slices from which said MR data are simultaneously acquired. 
     
     
       8. A method as claimed in  claim 1  comprising, when reconstructing said image data, separating the acquired measurement data in k-space into at least one slice measurement dataset for each of the slices from which said MR data were simultaneously acquired. 
     
     
       9. A method as claimed in  claim 8  comprising implementing said separation using a parallel acquisition technique. 
     
     
       10. A method as claimed in  claim 8  comprising entering the acquired MR data into k-space so that each slice measurement dataset is acquired in said direction of k-space, in which said measurement points are displaced by said additional phase, is complete according to the Nyquist criterion and, in said computer, extracting an associated slice measurement dataset from the respective slice measurement dataset by applying a Fourier transform thereto in a direction corresponding to said shift. 
     
     
       11. A method as claimed in  claim 8  comprising reconstructing said image data by separating the acquired measurement data into at least two slice measurement datasets for each of said slices, and averaging at least two of said slice measurement datasets of at least one slice, or averaging the image data reconstructed from the slice measurement datasets of at least one slice. 
     
     
       12. A magnetic resonance (MR) apparatus comprising:
 an MR data acquisition scanner; 
 a computer configured to emit control signals to said MR data acquisition scanner in order to cause the MR data acquisition scanner to execute an MR data acquisition sequence in which acquired MR data are entered into a memory organized as k-space along a k-space trajectory, and in order to radiate at least one multiband radio-frequency (RF) excitation pulse that selectively and simultaneously excites nuclear spins in at least two non-overlapping slices of an examination subject situated in the MR data acquisition scanner; 
 said computer being configured to determine phases that are to be additionally assigned to each of the simultaneously excited slices during execution of said MR data acquisition sequence, so as to displace at least a first of the measurement points for a respective slice to be displaced away from said k-space trajectory of the MR data acquisition sequence, in a direction in k-space corresponding to the additionally assigned phase for that respective slice; 
 said computer being configured to operate said MR data acquisition scanner to execute said MR data acquisition sequence with the assignment of the determined additional phases in order to acquire MR data from the respective slices with at least two RF reception coils; 
 said computer being configured to emit control signals to operate said MR data acquisition scanner in order to repeat acquisition of said MR data with said at least two RF reception coils in said MR data acquisition sequence with the phases that are additionally assigned being changed from repetition-to-repetition so that, compared with a previous acquisition of said MR data, at least a second of the measurement points in a respective repetition of the MR data acquisition sequence, that have not yet been displaced away from said k-space trajectory in previous repetitions, are displaced away from k-space trajectory in said direction in k-space, with said repetitions of said MR data acquisition sequence being repeated with a number of repetitions that causes at least one central region of k-space to be sampled completely by the repeated MR data acquisitions; 
 said computer being configured to determine a calibration dataset from the measurement data acquired in said central region of k-space; and 
 said computer being configured to reconstruct image data for the simultaneously excited slices from the acquired measurement data using said calibration dataset, and to make the image data available from the computer in electronic form, as a data file. 
 
     
     
       13. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a computer of a magnetic resonance (MR) apparatus that comprises an MR data acquisition scanner, and said programming instructions causing said computer system to:
 emitting control signals to said MR data acquisition scanner in order to cause the MR data acquisition scanner to execute an MR data acquisition sequence in which acquired MR data are entered into a memory organized as k-space along a k-space trajectory, and in order to radiate at least one multiband radio-frequency (RF) excitation pulse that selectively and simultaneously excites nuclear spins in at least two non-overlapping slices of an examination subject situated in the MR data acquisition scanner; 
 determine phases that are to be additionally assigned to each of the simultaneously excited slices during execution of said MR data acquisition sequence, so as to displace at least a first of the measurement points for a respective slice to be displaced away from said k-space trajectory of the MR data acquisition sequence, in a direction in k-space corresponding to the additionally assigned phase for that respective slice; 
 operate said MR data acquisition scanner to execute said MR data acquisition sequence with the assignment of the determined additional phases in order to acquire MR data from the respective slices with at least two RF reception coils; 
 emit control signals to operate said MR data acquisition scanner in order to repeat acquisition of said MR data with said at least two RF reception coils in said MR data acquisition sequence with the phases that are additionally assigned being changed from repetition-to-repetition so that, compared with a previous acquisition of said MR data, at least a second of the measurement points in a respective repetition of the MR data acquisition sequence, that have not yet been displaced away from said k-space trajectory in previous repetitions, are displaced away from k-space trajectory in said direction in k-space, with said repetitions of said MR data acquisition sequence being repeated with a number of repetitions that causes at least one central region of k-space to be sampled completely by the repeated MR data acquisitions; 
 determine a calibration dataset from the measurement data acquired in said central region of k-space; and 
 reconstruct image data for the simultaneously excited slices from the acquired measurement data using said calibration dataset, and make the image data available from the computer in electronic form, as a data file.